Method for producing a component by reshaping a plate, and device for carrying out said method

ABSTRACT

The invention relates to a method for producing a component by reshaping a coated plate consisting of tempering steel, said plate being austenitized before the reshaping by means of a first heat treatment, followed by the growth of the layer thickness. The aim of the invention is to optimize the process and to prevent the production of scrap plates caused by interruptions to the process. To this end, after being rapidly cooled, the heat-treated plate is temporarily stored, and is briefly heated again to the austenitization temperature, directly before being reshaped to form the component. Once the structure has been modified, the plate is reshaped and hardened. Preferably, the plate is heated a second time by induction.

The invention relates to a method for producing a component by reshapinga coated, preferably aluminum-coated plate of quenched and temperedsteel as claimed in the preamble of claim 1. The invention furthermorerelates to a device for carrying out said method.

The prior art generally discloses various forming processes for sheetbars from quenched and tempered steel in conjunction with hardeningprocesses. In a so-called “direct” forming process a flat sheet bar ofquenched and tempered steel is austenitized in a furnace, preferably acontinuous furnace, in a protective gas atmosphere. For example, aquenched and tempered steel of 22MnB5 grade which is annealed forseveral minutes at approximately 950° C. for austenitizing can be used.Then the hot, austenitized flat sheet bar is inserted with a preferablyautomated transfer means into a forming/tempering tool which is cooledfor serial processes. This tool is a component of a press and when thelatter is closed, the hot sheet bar is formed into a component of thefinal shape and in the closed tool, when the closing force is applied,it is cooled relatively quickly and thus hardened. The hardenedcomponent is removed from the tool, and if it is uncoated sheet, it isdescaled in a cleaning step, for example by sand blasting or shotpeening (this is not essential for coated components, since for examplealuminized sheets offer sufficient corrosion protection and scaling isprevented). This is followed by finish contour and hole trimming of thefinish-formed and hardened component, preferably by means of lasercutting. Mechanical cutting in a so-called press combination is alsoconceivable.

During heat treatment in the furnace, for example an aluminum coatingapproximately 25 μm thick in the initial state undergoes growth in layerthickness to approximately 45 μm, and directly bordering the basematerial of the sheet bar an AlSi layer with iron diffused therein isformed therefrom, which layer bears a relatively hard and brittle AlSilayer also forming which performs the actual anticorrosion function.

A typical process sequence (hot forming curve 20) with respect to heattreatment of the sheet bar in the course of its forming is shown forexample in FIG. 1 in a time-temperature diagram. Depending on the gradeof the quenched and tempered steel used, sheet metal thickness, initiallayer thickness, etc., the values given in FIG. 1 are of course subjectto certain fluctuations (lower/upper heat treatment boundary 18, 19).Thus it is easily conceivable for the sheet bar to be in the furnaceover a residence time of up to 30 minutes.

The furnace used is often a so-called continuous furnace with mold nestsor sheet bar receivers, or a grate pusher-type furnace with grates whichcarry the sheet bars and heat them within approximately 2 minutes bymeans of gas burners to the austenitization temperature and then keepthem at this temperature for several minutes by means of electricalheating. The advantage of the gas burner is higher output, converselyconventional electrical heating can be better controlled.

As shown in FIG. 1 the sheet bar is heated in the furnace to a targettemperature of approximately 950° C. and kept at this temperature.Austenitization is done at temperatures above approximately 720° C.Conventionally the residence time in the furnace is approximately 9minutes, the sheet bars within the first two minutes heating to thetarget temperature, while in the following approximately 7 minutesregranulation of the base material from a cubic-space centeredferrite-perlite structure into cubic surface-centered austenite which isnecessary for hardening takes place. In addition, the indicated timeinterval is also important mainly to achieve sufficient growth of theALSi protective layer.

In particular, with respect to the minimum annealing temperature andmaximum residence time of the sheet bar in the furnace there arecertain, more or less narrow limits, within which the process deliversquality components, i.e., that the sheet bars removed from the furnacecan still be used for the forming process and further applications. If aproblem arises in the continuing process, whether when a sheet bar isremoved from the furnace and further transported to theforming/tempering tool or within the finish contour and hole trimmingstation, for the duration of the problem no more sheet bars can beremoved from the furnace, the maximum allowable residence time isgenerally exceeded, and all the sheet bars in the furnace are scrap andmust be disposed of.

The object of the invention is to develop the generic process forproducing a component by forming a coated sheet bar of quenched andtempered steel such that the process sequence can be optimized and thatespecially the costly formation of scrap sheet bars can be prevented ifprocess disruptions arise.

This object is achieved with respect to the process with the otherfeatures according to the characterizing part of claim 1 and withrespect to the device for carrying out the process as claimed in theinvention with the features of claim 10.

The advantages of the procedure as claimed in the invention aremanifold. Thus, now there is no longer any relationship with respect tothe residence time of the sheet bars within the furnace and problems inthe process outside the furnace. By decoupling the sequences for theactual process of sheet bar forming, the amount of area required andinfrastructure are less. Buffering/intermediate storage of the quenchedand tempered sheet bars is possible so that heat treatment, to influencethe AlSi layer, additionally can easily take place at the steelmanufacturer or sheet metal supplier.

This upstream heat treatment removed from storage is already known, asfollows from EP 0 946 311 B1 and DE 102 12 400 C1. Annealing treatmentby means of induction heating viewed in itself is also already priorart, as is also mentioned for example in the latter document.

Advantageous embodiments and developments of the invention are claimedin the dependent claims.

The invention will be detailed below using the exemplary embodiment.

FIG. 1 shows the process sequence as claimed in the invention forproducing a component by forming a sheet bar,

FIG. 2 shows the temperature-time diagram of the first sheet bar heattreatment, and

FIG. 3 shows the temperature-time diagram of the second sheet bar heattreatment.

FIG. 1 schematically shows the process sequence as claimed in theinvention for producing a component 5 by forming a coated sheet bar 1 ofquenched and tempered steel by means of a device 2 suited for thispurpose. From the steel delivered in the rolled state, a so-called coil3, by means of a tool 4 the steel is unrolled, flattened, and the sizeof the sheet bar 1 necessary for the finished component 5 is punched outor cut to size. From there the sheet bars 1 are supplied to a bufferzone 6. This intermediate storage is not absolutely necessary, ratherthe sheet bars 1 can also be supplied directly after leaving the tool 4to a first furnace 7 in which they undergo heat treatment according tothe temperature-time diagram shown in FIG. 2. Directly downstream fromthe first furnace 7 is a cooling zone 8 in which the sheet bars 1 arequenched and pass through the concluding phases of heat treatment.Leaving the cooling zone 8, the quenched and tempered sheet bars 1 aresupplied to an intermediate storage 9.

The first furnace 7 can be the aforementioned continuous furnace, arevolving furnace, or the like in terms of its structural design.

The individual phases of heat treatment were explained in theintroductory section with reference to FIG. 2. The relatively slowheating to the target temperature and the remaining residence time inthe first furnace 7 for inducing austenitization and for changing thetopography (coating structure, layer thickness) add up to the totalresidence time of approximately 9 minutes, and empirically a maximumresidence time of 30 minutes should not be exceeded, so that the sheetbar does not become unusable. The transport into the cooling zone 8 andthe quenching of the sheet bar 1 there take place within relativelyshort time intervals, while the remaining cooling to room temperature RTcan take place in the intermediate storage 9. At the end of heattreatment the sheet bar 1 has a martensitic structure.

By means of a suitable transport device 10, for example an articulatedarm robot, the sheet bars 1 are supplied to an induction furnace 11,from where they are inserted into a cooled forming/tempering tool 13suitable for serial processes by way of another transport device 12, forexample in turn an articulated arm robot. A press means 14 and a coolingdevice 15 are assigned to the tool 13, and when the press means 14 isclosed the hot sheet bar 1 is formed into a component 5 with the finalshape and is rapidly cooled and hardened in the closed forming/temperingtool 13 with the closing force applied. In the last process step eachcomponent 5 is supplied via a transport means 16 to a trimming device17, where finish contour and hole trimming of the completely formed andhardened component 5 is done preferably by means of laser cutting. Ofcourse this can also take place mechanically by way of suitable trimmingblades.

The heat treatment of the sheet bar 1 which takes place in the inductionfurnace 11 and in the downstream forming/tempering tool 13 isillustrated in the temperature-time diagram as shown in FIG. 3 using thehot forming curve 20 and lower/upper heat treatment boundary 18, 19. Itis characterized by an extremely short residence time of the sheet barin the induction furnace 11. While heating to the target temperature(austenitization temperature) takes place within a few seconds(approximately ten seconds), a downstream short residence time ofapproximately ten seconds up to a maximum two minutes is used to allow astructure transformation to take place. A change of thickness andstructure of the coating is no longer necessary since this has alreadytaken place in the first furnace 7. After an accordingly extremely shortresidence time in the induction furnace 11, the sheet bar can besupplied to the forming/tempering tool 13 in which in addition toforming, quenching takes place in the same manner (same behavior of thehot forming curve 20) as in the cooling zone 8. When leaving theforming/tempering tool 13 the component 5 already has a martensiticstructure, cooling to room temperature RT can take place upon furthertransport to or within the trimming device 17.

In this way a sheet bar 1 having an original tensile strength ofapproximately 500 to 600 N/mm² is formed into a component 5 having atensile strength of approximately 1300 to 1500 N/mm².

In an advantageous development of the invention it would be conceivableto heat the sheet bar 1 in the induction furnace 11 in part to differentintensities, with the result that if desired the formed and quenchedcomponent 5 has partially different strengths.

Furthermore, it would be possible preferably to locally reinforce thesheet bar 1 before the second heat treatment (induction furnace 11) forexample by welding on reinforcing sheets (patches). A composite sheetpatched in this way could then be sent to the second furnace andafterwards to the forming/tempering tool 13. This would have altogetherpositive effects on the material properties and accuracy of shape.

The process as claimed in the invention can also be advantageously usedwhen using tailored blanks as sheet bars.

One important advantage of the invention is the possibility ofdecoupling the individual process steps. Thus the first heat treatmentin the furnace 7 can take place at the steel or sheet manufacturer andthe sheet bars 1 pretreated in this way can then be made available tothe processing company (for example, motor vehicle manufacturer)(intermediate storage 9).

In another advantageous development of the invention it is conceivableto assign an inductor to the transport device 10 and to structurallyintegrate it into the transport device 10, so that the heat treatment ofthe sheet bar 1 can proceed during its transport to theforming/tempering tool 13. A separate induction furnace 11 and anothertransport device 12 downstream from it can thus be eliminated.

1. A method for producing a component by reshaping a coated plate ofquenched and tempered steel, before reshaping in a first process step,the plate being supplied to a first furnace and being austenitized thereand a residence time of the plate in the first furnace being chosen suchthat in addition to the structure transformation an increase in thelayer thickness takes place, the method comprising: rapid cooling andsubsequent intermediate storage of a heat treated sheet bar, repeated,brief heating of the sheet bar in a second furnace to an austenitizationtemperature directly prior to forming into the component, and formingand hardening the sheet bar after completed structure transformation. 2.The method as claimed in claim 1, wherein the residence time in thefirst furnace is between nine minutes and thirty minutes.
 3. The methodas claimed in claim 1, wherein when the sheet bar is heated again to anaustenitization temperature in the second furnace, such that only onestructure transformation takes place, but no longer an increase of layerthickness.
 4. The method as claimed in claim 3, wherein a residence timeof the sheet bar in the second furnace is from ten seconds to two andone half minutes.
 5. The method as claimed in claim 1, wherein the sheetbar is heated in the first furnace by electricity or gas, while heatingin the second furnace takes place by induction.
 6. The method as claimedin claim 1, wherein the first heating takes place at a steel or sheetmanufacturer, while the second heat treatment takes place at aprocessing company.
 7. The method as claimed in claim 1, wherein duringthe second heat treatment the sheet bar is heated to differenttemperatures over its surface.
 8. The method as claimed in claim 1,wherein the sheet bar, before reheating in the second furnace, islocally reinforced by applying at least one reinforcing sheet.
 9. Theprocess as claimed in claim 1, wherein the sheet bar is a tailoredblank.
 10. A device for carrying out the process of claim 1, the devicecomprising: a tool for producing sheet bars from a coil, a first furnacefor initial heat treatment including inducing an increase in the layerthickness of the sheet bars, a cooling zone for the sheet bars, anintermediate storage area for the sheet bars, a second furnace forrepeated heat treatment of the sheet bars, a forming/tempering tool witha press means and a cooling device, a trimming device for producing atrimmed finish contour and holes.
 11. The device as claimed in claim 10,wherein the first furnace is an electricity-based and/or gas-basedfurnace (7), and the second furnace is an induction furnace.
 12. Thedevice as claimed in claim 11, wherein an inductor is integrated intothe transport device which is located between the intermediate storageand the forming/tempering tool.
 13. The device as claimed in claim 10,further comprising a station for applying at least one reinforcing sheetto the sheet bar between the cooling zone and the second furnace. 14.The method of claim 1 wherein the plate is an aluminum-coated plate. 15.A method of producing a component comprising: providing a coated sheetof quenched and tempered steel; shaping the sheet; heat treating theshaped sheet to perform a first austenitization of the sheet, duringwhich a structure transformation occurs increasing the layer thicknessthereof; cooling and quenching the heat treated sheet; storing thecooled and quenched sheet; performing a second heat treatment to performa second austenitization of the sheet; and forming the sheet into thecomponent.
 16. The method of claim 15 wherein the first heat treatmentis performed in a gas or electric furnace and/or the second heattreatment is performed in an induction furnace.
 17. The method of claim15, further comprising reinforcing the sheet between the heat treatmentsteps.
 18. The method of claim 15 wherein the first heat treatmentcomprises a residence time of between approximately 9 and 30 minutes.19. The method of claim 15 wherein second heat treatment comprises aresidence time of between approximately 10 seconds to two minutes. 20.The method of claim 15 wherein during the second heat treatment step, nochange in thickness of the sheet occurs.
 21. The method of claim 15wherein the sheet has a martensitic structure following the formingstep.
 22. The method of claim 15, further comprising transporting thesheet from a first location, where the first heat treatment isperformed.
 23. The method of claim 22 wherein the second heat treatmentis performed during the transporting.
 24. The method of claim 15 whereinthe second heat treatment comprises heating the sheet to differentintensities at different locations thereof.
 25. A product produced bythe method of claim 15.